The mechanisms that control IgE antibody production are under study utilizing a model system in mice. Current emphasis is on the measurement of specific IgE produced on challenge by antigen. An assay for quantitating total IgE production is under development. Studies are under way to determine the genetic control mechanisms that determine the amount of IgE produced. Using in vivo cell reconstitution experiments we are studying the defect involved in a strain of mice that is deficient in IgE production.